1. Field of the Invention
The present invention relates to a semiconductor laser having a double heterostructure and, more particularly, to a distributed feedback (DFB) semiconductor laser in which a diffraction grating formed along a optical waveguide produces optical feedback.
2. Description of the Related Art
Various types of semiconductor lasers have recently been used as light sources for optical communication and optical information processing. One of the semiconductor lasers is a distributed feedback (DFB) semiconductor laser having a diffraction grating along its optical waveguide. As been derived using coupled-mode analysis, the DFB laser oscillates at a single wavelength (in a single longitudinal mode) because of wavelength selectivity of the grating. The DFB laser is particularly improved in practicability by using GaInAsP/InP material system for a light source for long-distance high-speed optical communication.
In a conventional semiconductor laser, such as a FabryPerot (FP) type semiconductor laser, optical feedback is produced by both facets of the laser which serve as reflecting mirrors. In contrast, optical feedback in the DFB semiconductor laser is produced mainly by the diffraction grating having wavelength selectivity, so that principally the facets are not necessary. In the DFB semiconductor laser, however, a single longitudinal mode capability depends upon a shape and height of the diffraction grating, the reflectivity at the facets which are inevitably formed, and the phase of the diffraction grating at the facets.
An advanced laser structure has recently been noted in which reflectivities of both cleaved facets are reduced and the phase of the grating is discontinuous at the cavity center (for example, a phase-shift corresponding to 1/4 of guided wavelength .lambda.). The DFB laser with such a structure has a great difference in threshold gain between the lasing longitudinal mode and the other modes. This type of DFB lasers therefore are remarkably advantageous to a single longitudinal mode operation. This laser however has a problem of spatial holeburning along its axis direction (Soda et al., "Stability in Single Longitudinal Mode Operation in GaInAsP/InP Phase-Adjusted DFB Lasers," IEEE JOURNAL OF QUANTUM ELECTRONICS, Vol. QE-23, No. 6, Jun., 1987). More specifically, when a normalized coupling coefficient .kappa. L is more than 1.25, the power of guided waves concentrate at a position of the .lambda./4 phase-shift. The optical intensity profile along the axial direction causes a distribution of the carrier density in an active layer. The refractive index of an optical waveguide is distributed, corresponding to the distribution of the carrier density profile. The variation in the refractive index reduces a large gain difference, .DELTA..alpha. between longitudinal modes and thus greatly degrades the single longitudinal mode behavior.
The holeburning also causes the following disadvantage. If the refractive index of a region of the waveguide abruptly changes, the guided mode cannot efficiently be transformed and a part of light energy is radiated as a radiation mode outside the optical waveguide. The beams of radiation mode then interferes with an output light beam emitted from an output facet. A ripple is formed by the interference in the output radiation pattern, namely, a far field pattern (FFP). Therefore, the output light beam cannot efficiently be coupled with other optical components.
FIGS. 7A and 7B are plan views of a conventional laser and views showing the FFP (=.theta. //) of the output light beam. In FIGS. 7A and 7B, .theta. // indicates a full angle at half maximum in the horizontal direction of the FFP, and the optical intensity profiles (I) of guided wave in the cavity direction are shown on the right of the plan views of the laser.
FIG. 7A shows a Fabry-Perot type semiconductor laser. In this laser, the optical intensity profile I is relatively smooth and thus an undesired interference does not occur between the output light beam and radiation mode. The FFP (.theta. //) therefore has an ideal shape without large ripples.
FIG. 7B shows a .lambda./4 phase shifted type DFB laser. In this laser, the guided waves concentrate at a .lambda./4 phase shifted position 30 (I). The irregularity of optical intensity profile along the axial direction varies the carrier density in the active layer. As described above, in a portion of the optical waveguide where its refractive index greatly changes, the guided mode cannot efficiently be transformed and part of optical energy is radiated as a radiation mode outside the guiding layer. The FFP (.theta. //) is degraded by an interference between a radiation mode 31 and an output beam 32, as shown in FIG. 7B.
Not only the .lambda./4 phase shift type DFB laser but also an usual DFB laser has a drawback wherein the FFP of an output light beam of the laser is easily degraded ordered by a slight variation in refractive index.